Liquid crystal substances are applied to display media in which the reversible movement of liquid crystal molecules is made use of, such as display devices typically of TN or STN mode. Besides this application, liquid crystal substances have been studied for application to optically anisotropic elements, such as a retardation film, a polarizer, a polarizing prism, and various optical filters, taking advantage of their anisotropy in physical properties, such as refractive index, dielectric constant, and magnetic susceptibility, as well as their alignment properties.
Such an optically anisotropic element is obtained by, for example, uniformly aligning the molecules of a liquid crystal compound having a polymerizable functional group or a composition containing the liquid crystal compound into a liquid crystal phase and irradiating the compound or the composition being in the liquid crystal phase with energy rays, such as ultraviolet rays, to cause photopolymerization to form a polymer film in which the uniformly aligned state is semi-permanently fixed.
When the composition forming a polymer film has a high liquid crystal phase transition temperature, photopolymerization induced by energy rays may unintentionally invite thermal polymerization, which disturbs the uniform alignment of the liquid crystal molecules, making it difficult to fix a desired state of alignment. In order to facilitate temperature control during cure, a polymerizable composition showing a liquid crystal phase at or near room temperature is demanded.
The polymer film is obtained by polymerizing the polymerizable composition in the form of coating film applied to a substrate. If the composition contains a non-polymerizable compound, the resulting polymer film may have insufficient strength or contain residual stress-induced strain. Removing a non-polymerizable compound using, e.g., a solvent can result in a failure to retain film homogeneity and cause unevenness. To obtain a polymer film with a uniform thickness, it is therefore preferred to apply a polymerizable composition in the form of a solution in a solvent to a substrate. Hence, it is desirable for a liquid crystal compound or a composition containing it to have good solubility in a solvent.
Patent Document 1 discloses an optically anisotropic element obtained by polymerizing a nematic composition. The nematic composition has a problem that the film immediately after cure is uniform in thickness but poor in heat resistance and solvent resistance and, with time after cure, undergoes deformation or reduction in optical characteristics.
The present inventors have previously proposed compositions having excellent properties in terms of heat resistance, solvent resistance, solvent solubility, high glass transition temperature, and low liquid crystal phase transition temperature in Patent Document 2, Patent Document 3, Patent Document 4 and Patent Document 5.
In general, formation of a thick film from a polymerizable composition containing a liquid crystal compound encounters with difficulty in controlling molecular alignment of the liquid crystal compound, which can cause problems, such as reduction in transmittance and coloration. On the other hand, a thin polymer film having satisfactory molecular alignment over the entire area thereof can be obtained, but formation of a thin film has difficulty in thickness control, readily resulting in nonuniform surface condition or crystallization. Thin polymer films obtained from known compositions are thus unsatisfactory in physical properties and optical characteristics.    Patent Document 1: JP 8-3111A    Patent Document 2: JP 10-87565A    Patent Document 3: JP 11-130729A    Patent Document 4: JP 2005-263789A    Patent Document 5: JP 2005-309255A